+ cation|221x221px|left
One-electron oxidation The reactivity of [{(η5- Me5C5)Mo}2(μ,η6-P6)] toward
silver and
copper monocationic
salts of the weakly coordinating anion [Al{OC(CF3)3}4]− ([TEF]) was studied by Fleischmann et al. in 2015. Addition of a solution of Ag[TEF] or Cu[TEF] to a solution of [{(η5- Me5C5)Mo}2(μ,η6-P6)] in
chloroform results in
oxidation of the complex, which can be observed by an immediate colour change from amber to dark teal. The
magnetic moment of the dark teal
crystals determined by the
Evans NMR method is equal to 1.67 μB, which is consistent with one unpaired electron. Accordingly, [{(η5- Me5C5)Mo}2(μ,η6-P6)]+ is detected by
ESI mass spectrometry. The crystal structure of the teal product shows that the triple‐decker geometry is retained during the one‐electron
oxidation of [{(η5- Me5C5)Mo}2(μ,η6-P6)]. The Mo—Mo bond length of the [{(η5- Me5C5)Mo}2(μ,η6-P6)]+ cation is 2.6617(4) Å; almost identical to the bond length determined for the unoxidized species at 2.6463(3) Å. However, the P—P bond lengths are strongly affected by the
oxidation. While the P1—P1′ and P3—P3′ bonds are elongated, the remaining P—P bonds are shortened compared to the average P—P bond length of about 2.183 Å in the unoxidized species. Therefore, the middle deck of the 27 valence electron [{(η5- Me5C5)Mo}2(μ,η6-P6)]+ complex can best be described as a bisallylic distorted P6 ligand, intermediate between the 28 valence electron complexes with a perfectly planar symmetrical ring, and those with 26 valence electrons displaying a more amplified in-plane distortion.
Density functional theorem (DFT) calculations confirm that this distortion is due to depopulation of the P
bonding orbitals upon oxidation of the triple-decker
sandwich complex.
Cu[TEF] & Ag[TEF] File:Reactivity of (((η5- Me5C5)Mo)2(μ,η6-P6)).png|thumb|Reactivity of [{(η5- Me5C5)Mo}2(μ,η6-P6)] towards the
cations Cu+, Ag+, and Tl+|299x299px To avoid
oxidation of [{(η5- Me5C5)Mo}2(μ,η6-P6)], further reactions were performed in
toluene to decrease the
redox potential of the cations. This resulted in a bright orange coordination product upon reaction with
copper, although a mixture also containing the dark teal
oxidation product was obtained upon reaction with
silver.
Single‐crystal X‐ray analysis reveals that this product displays a distorted
square‐planar coordination environment around the central cation through two side‐on coordinating P—P bonds. The Ag—P distances are approximately 2.6 Å, whereas the Cu—P distances are determined to be approximately 2.4 Å. The P—P bonds are therefore elongated to 2.2694(16) Å and 2.2915(14) Å upon coordination to
copper and
silver, respectively, whilst the remaining P—P bonds are unaffected. In another experiment Cu[TEF] is treated with [{(η5- Me5C5)Mo}2(μ,η6-P6)] in pure
toluene and the solution shows the bright orange color of the complex cation [Cu([{(η5- Me5C5)Mo}2(μ,η6-P6)])2]+. However, analysis of crystals from this solution reveals a distorted
tetrahedral coordination environment around Cu. The resulting Cu—P distances are somewhat shorter than their counterparts discussed above. The coordinating P—P bonds are a little longer, which is attributed to less steric crowding in the
tetrahedral coordination geometry around the Cu center. The successful isolation of [Cu([{(η5- Me5C5)Mo}2(μ,η6-P6)])2]+ either as its tetrahedral or square‐planar isomer is therefore achievable.
DFT calculations show that the
enthalpy for the tetrahedral to square‐planar
isomerization is positive for both metals, with the tetrahedral coordination being favored. When
entropy is taken into account, small positive values for Cu+ and larger, but negative, values for Ag+ are observed. This means that the tetrahedral geometry is predominant for Cu+, but a significant percentage of the complexes adopt a square‐planar geometry in solution. For Ag+, the equilibrium is shifted significantly to the right side, which is presumably why a tetrahedral coordination of [{(η5- Me5C5)Mo}2(μ,η6-P6)] and Ag+ has not yet been observed. Examination of the
crystal packing reveals that these products are layered compounds that crystallize in the
monoclinic C2/
c space group with alternating negatively charged layers of the [TEF] anions and positively charged layers of isolated [M([{(η5- Me5C5)Mo}2(μ,η6-P6)])2]+ complexes. The layers lie inside the
bc plane, alternate along the
a axis, and do not form a two‐dimensional network.
Tl[TEF] The treatment of [{(η5- Me5C5)Mo}2(μ,η6-P6)] with Tl[TEF] in
chloroform gives an immediate color change from amber to a deep red. The crystal structure reveals a
trigonal pyramidal coordination of the
thallium cation, Tl+, by three side‐on coordinating P—P bonds of the P6
ligands. Two of these P6
ligands show shorter and uniform Tl—P distances of 3.2–3.3 Å with P—P bonds elongated to about 2.22 Å, whilst the third unit shows an unsymmetrical coordination with long Tl—P distances of approximately 3.42 and 3.69 Å and no P—P bond elongation. of a) [Ag([{(η5- Me5C5)Mo}2(μ,η6-P6)])2]+and b) [Tl([{(η5- Me5C5)Mo}2(μ,η6-P6)])2]+ showing the alternation of anionic and cationic layers along the a axis. Tl+ positions are half‐occupied.|350x350px Although the environment of Tl+ is distinctly different from that of Cu+ and Ag+, their structures are related by the two‐dimensional
coordination network that propagates inside the
bc plane. Crucially, whilst Cu+ and Ag+ form layered structures with isolated [M([{(η5- Me5C5)Mo}2(μ,η6-P6)])2]+ complex cations, there is a
statistical distribution of the Tl+ cations inside the two‐dimensional coordination, which shows further interconnection of the P6
ligands to form an extended 2D network that could be regarded as a
supramolecular analogue of
graphene. == Jahn–Teller distortion ==